A receiver apparatus (receiver) for receiving radio waves of a mobile phone, a wireless LAN (local area network), terrestrial digital broadcasting and so on receives not only a direct wave which is an incoming wave that directly arrives from a transmitter, but also a delay wave which is an incoming wave that arrives by being reflected and scattered by an obstacle such as a building and a structure. A communication environment in which the radio waves transmitted from the transmitter arrive at the receiver through various routes as described above is referred to as a multipath environment.
Directional characteristic control of an array antenna including a plurality of antenna elements is known as one of the technologies for suppressing degradation of reception performance in the multipath environment. In the array antenna, a directional characteristic can be given to the array antenna by controlling a plurality of weight coefficients that are used when a plurality of signals received through the plurality of antenna elements are synthesized. In the multipath environment, the degradation of the reception performance due to the influence of the delay wave can be suppressed by controlling the directional characteristic of the array antenna so that a main lobe of the array antenna is directed in an arrival direction of the direct wave (direction of the transmitter). It is necessary to accurately estimate an arrival angle of the direct wave, in order to improve the reception performance by the directional characteristic control of the array antenna.
In a case where the receiver is fixed and the direction of the transmitter is already known, the directional characteristic of the array antenna may be adjusted manually so that the main lobe of the array antenna is directed to the transmitter. However, in a case where the radio waves are received by the receiver in a moving object such as a car, the direction of the transmitter seen from the receiver changes, and thus it is necessary to estimate the arrival angle of the direct wave from a reception signal in which the direct wave and the delay wave are superposed. Incidentally, radio wave environments of wireless communication are classified into LOS (Line Of Sight) in which the transmitter exists in a field of view seen from the receiver and NLOS (None Line Of Sight) in which the transmitter exists outside the field of view seen from the receiver. The present application estimates the arrival angle of the direct wave, assuming the LOS environment.
In general, a MUSIC (MUltiple SIgnal Classification) method and an ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) method that use an eigenvalue and an eigenvalue vector of a correlation matrix of a signal received by the array antenna including N (N is an integer equal to or greater than 2) antenna elements are known as methods for measuring the arrival angle of the radio wave.
However, the number of incoming waves that can be measured by the MUSIC method or the ESPRIT method is limited, and is N waves at the maximum. In a case where the MUSIC method or the ESPRIT method is used under the multipath environment in which there are a large number of delay waves that arrive by being reflected by surrounding obstacles, in addition to the direct wave that directly arrives at the receiver from the transmitter, it is necessary for the antenna array to include the antenna elements of the same number as the maximum number of assumed incoming waves (direct wave and delay waves), and there is a drawback that the array antenna becomes too large.
Patent reference 1 describes a method for estimating the arrival angles of 3 or more waves by two antenna elements. In this method, the incoming waves having different delay times are separated by using complex delay profiles calculated from signals received by respective antenna elements, and the arrival angles of the incoming waves are estimated on the basis of the phase difference between the antenna elements of each separated incoming wave.
Patent reference 2 and patent reference 3 describe a method for estimating the arrival angle by utilizing multibeam by an array antenna. In this method, the arrival angles of incoming waves larger in number than the antenna elements can be estimated. In this method, first, the incoming waves having different arrival angles are separated by using a plurality of beamformings having different main lobe directions, and thereafter delay times are estimated on the basis of delay profiles calculated from the results of the respective beamformings. A rough arrival angle can be estimated from the direction of the main lobe of the beamforming of the largest electric power. Moreover, estimation accuracy of the arrival angle can be further improved by performing estimation of the arrival angle on the basis of the electric power difference between the beamformings whose main lobe directions are adjacent.